1. Field of the Invention
The invention relates to a photovoltaic solar cell with an electrical solid material contact between a semiconductor layer of a layer thickness of dHL and with a plurality of metal nano emitters embedded in an electrically insulating oxide layer disposed on the semiconductor layer and each provided with a space-charge region of dimension w within the semiconductor layer to which the minority carriers migrate over a diffusion length L, and with a transparent conductive layer electrically insulated from the semiconductor layer by the oxide layer as well as with front and rear contacts, and to a method of manufacturing such a solar cell.
2. The Prior Art
In photovoltaic current generation the efficiency of the conversion of impinging sun light to electrical current is of the utmost importance. In order to obtain an optimum yield, aside from reducing losses of reflection and recombination, particular efforts are made to reduce losses from shadows on the light irradiation surface and to improve the collection of light. Shadows on the light irradiation surface result especially from the front contacts, the transparent electrically conductive layer, for instance ITO, and from the emitter layer in the case of Schottky-type solar cells with a solid material contact between the doped semiconductor layer and a rectifying metal layer. The same is true of simple p-n-transitions between a normally doped and a highly doped semiconductor layer. For purposes of an efficient photovoltaic conversion of energy between solid material contacts the emitter layer may be applied as a very thin uninterrupted layer on the semiconductor layer operating as the base or absorber. This may be done by vacuum processes, such as PVD, and diffusion processes. Furthermore, dot-contact solar cells have been proposed which on their rear surface are provided with an optimized dot-contact pattern in direct contact with highly doped insertion zones in the absorber layer (see publication I by J. Zhao et al.: “22.7% efficient pearl silicon solar cell module with a textured front surface”, 26th PVSC, 1197, Anaheim, Calif., pp. 1133-1136, FIG. 2). From this publication, pyramidal structuring of the cell surface in the region of the uppermost layers, including the emitter layer, for the prevention of refections, are also known. However, in the area of the of this surface structuring the depth of penetration of the emitter layer into the semiconductor layer of correspondingly conforming structure is relatively minor. The front contacts are applied as strips as narrow as possible in order to reduce losses from shadows and thus to increase the conversion efficiency of the solar cell. The emitter layer is applied as a closed surface and is more highly doped beneath the contact strips than in the area of directly impinging sunlight. Furthermore, for purposes of reducing shadow losses it is known from WO 02/103810 A1 to structure the front contact as wedge-like troughs which are narrower than simple contact strips but which have a greater depth of penetration into the emitter layer and the semiconductor layer positioned below it (buried contact). However, no significant improvement in efficiency can be achieved in view of the fact that the main light collection takes place in the area of the contact-free emitter-absorber transition with increased depth of penetration. For improving the efficiency, it is known from Japanese abstract JP 04015963 A to insert individual emitter regions deeply into the absorber layer in order to prevent recombinations (buried insular regions). However, each emitter region is connected to a front contact strip by its own contact hole. A similar approach of the semiconductor material is known from DE 198 37 365 A1 which teaches the insertion of clusters into the active absorber layer for providing additional charge carriers. For using additional wavelength ranges of the sunlight this leads to changing the optical properties of the semiconductor. However, the clusters grow with inherent defects and are thus difficult to control in terms of their distribution and size.
The state of the art upon which the present invention is based, is disclosed by Publication II by H. Tsubomura et al.: “Effect of Microscopic Discontinuity of Metal Overlayers on the Photovoltages in Metal-Coated Semiconductor-Liquid Junction Photoelectrochemical Cells for Efficient Solar Energy Conversion”. This publication aims primarily at improving the catalysis of the photo-chemical process in an electrolytic solar cell (PEC). The application of catalytically active metal films resulted in a solid material contact which, structured as a Schottky contact, is necessarily inferior to a p/n semiconductor contact since it leads to increased losses from charge carrier recombination. The quantity of the current losses occurring at the semiconductor-metal-contact may, however, be reduced by replacement of the continuous metal film by very small (diameter in the range of several nm) metal islands as metallic nano emitters which cover but a small portion of the surface. These metal dot contacts are fabricated by etching of a metal film on a silicon diode. However, in an aqueous electrolyte the latter immediately forms an oxidic insulating layer. In this manner, the semiconductor-electrolyte-contact is replaced by a pure solid material contact in which in addition to catalytically active metal islands an insulating oxide layer covers most of the surface. In discussing his publication, Tsubomura states that his approach of reducing current losses can be applied to a solid material solar cell if instead of the redox electrolytic solution a transparent conductive layer is applied to the oxide layer. However, Tsubomura makes no statements relating to dimensional rules regarding size and distribution of the metal islands. The islands are disposed on the surface of the semiconductor layer at a relatively large distance from each other (four times their diameter), so that the efficiency attainable particularly in solid material solar cells is not fully satisfactory.